Exemplary embodiments of the present invention relate to a device for storing low molecular gases under high pressure.
Devices for storing gases or compressed gas storage elements are generally known. Typically, such compressed gas storage elements are sub-divided into different types. For example, a so-called type II compressed gas storage element is a compressed gas storage element with an inner shell made of high strength steel and an outer sleeve surrounding the inner shell, the outer sleeve being composed, for example, of fiber reinforced material. A type III pressure storage element comprises an inner shell composed of aluminum and a type IV pressure storage element typically comprises an inner shell made of a plastic material, for example high density polyethylene (HDPE), which is surrounded by at least one outer sleeve composed of fiber-reinforced material.
Such compressed gas storage elements serve as devices for storing gases under high pressures. In general, pressures are provided in the magnitude of approximately 350 bar, in the magnitude of approximately 700 bar or also in the magnitude of 1100 to 1200 bar in order to be able to store, particularly in case of light gases, a quantity of gas that is as large as possible with a comparatively manageable volume of the device.
If low molecular gases, in particular hydrogen, are stored under high pressure in the device for storing gas, it is practically unavoidable that small quantities of the low molecular gas will diffuse through the device and flow into the environment. This is generally unproblematic due to the comparatively small quantity.
In certain situations, however, a certain quantity of hydrogen has diffused through the inner shell and remains in the region between the inner shell and the outer sleeve. During normal operation, a slow diffusion of this hydrogen through the outer sleeve will then occur so that no problems arise. In case of a virtually empty compressed gas storage element, however, significant problems can arise. During refilling, a pressure increase arises relatively quickly in the inner shell from a very low residual pressure with virtually empty compressed gas storage element to a very high pressure with a completely filled compressed gas storage element, for example to a pressure in the magnitude of the abovementioned 700 bar. As the inner shell typically has a certain elasticity, and here in particular if it is formed as a plastic inner shell in a type IV compressed gas storage element, the relatively quick pressure increase leads to the gas cushions located between the inner shell and the outer sleeve—the gas cushions being composed of the low molecular gas that has diffused through the inner shell—being placed under a relatively high pressure. Under these conditions, the gas then passes comparatively quickly through the outer sleeve so that a gas cloud forms in this situation in the environment of the outer sleeve, the concentration of the gas cloud being significantly higher than in case of diffusion through the walls of the compressed gas storage element in normal operation. If the leak-tightness of the compressed gas storage elements is determined via corresponding detectors, in particular the leak-tightness of the compressed gas storage elements used to store combustion gas in vehicles, this concentration is typically so high that an alarm is triggered and this generally leads to an emergency disconnection of the system, for example to an emergency cessation of filling.
German Patent Document DE 2008 039 573 A1 discloses a compressed gas storage element constructed with an inner shell and at least one outer sleeve. In the region between the inner shell and the at least one outer sleeve, a diffusion layer is arranged that is designed so that the low molecular gas that has possibly diffused through the inner shell can be purposefully collected and carried away via a connection element. As this removal can take place with comparatively low pressure loss and thus offers a significantly lower diffusion resistance than the penetration of the outer sleeve, low molecular gas that may have diffused through the inner shell will follow this path and thus be purposefully removed from the region of the diffusion layer.
The structure thereby has the disadvantage that it requires comparatively high resources and does not limit gas losses from the compressed gas storage element but instead even encourages the diffusion through the inner shell, as it provides a path for outflow of the gas which has diffused through the inner shell with very low pressure loss and thus accelerates the diffusion through the pressure drop.
Exemplary embodiments of the present invention provide a structure of a device for storing low molecular gas under high pressure that avoids the aforementioned disadvantages, minimizes the gas loss through diffusion, and in particular prevents the problem of an abruptly increasing gas concentration in the environment of the device during filling.
According to exemplary embodiments of the present invention thus provides a device comprises at least one valve means with backflow prevention means in the region between the inner shell and the outer sleeve, connecting this region between the inner shell and the outer sleeve to the inside of the inner shell. The backflow prevention means is designed so that gas diffusing through the inner shell can flow back into the inner shell under suitable pressure conditions. The construction with the backflow prevention means thus allows gases to be able to flow back from the region between the inner shell and the outer sleeve into the region within the inner shell. During normal operation of the device the unavoidable losses of hydrogen will arise through diffusion of the hydrogen through the inner shell and then possibly also the outer sleeve. Typically, during normal operation of the device the low molecular gas under pressure is removed from the region of the device so that the device or the region in the inner shell gradually becomes empty and loses pressure. If this situation arises and low molecular gas is present in the region between the inner shell and the outer sleeve, the gas having diffused through the inner shell, this will flow back into the inner shell through the valve means with backflow prevention means when the pressure in the inner shell is lower than the pressure in the region between the inner shell and outer sleeve. Two effects can thereby be achieved. First, at least a part of the low molecular gas that diffused through the inner shell is recovered insofar as it can flow back into the region of the inner shell if the device is empty or virtually empty. In addition, when the device is comparatively empty such a backflow is facilitated so that no, or only very little, low molecular gas is found between the inner shell and the outer sleeve in these operating conditions. If the device is now filled and the inner shell is thus impacted in a comparatively short time with a relatively high pressure, no or at best very little gas can be pressed through the outer sleeve so that an increase in the concentration of the low molecular gas in the environment of the device is securely and reliably avoided during filling.
The particular advantage of the structure thus lies in that safety-critical situations during filling due to gas leaving the region between the inner shell and outer sleeve are prevented and at the same time at least a part of the gas that diffused through the inner shell can flow back into the inside of the inner shell and can be used there. The backflow prevention means is thereby formed so that a through-flow from the region of the inner shell into the region between the inner shell and outer sleeve is prevented.
According to a particularly favorable and advantageous development of the device according to the invention, the at least one valve means is pre-tensioned via a spring means in the closed state. Such pre-tensioning via a spring means, which can be formed, for example, as a helical spring, pressure spring or a torsion spring at a suspension point of a flap or similar in such a way that it presses the non-return means of the valve means in support of the pressure of the gas in the inner shell into the closed state. Only when a clear pressure difference arises between the region between the inner shell and the outer sleeve on the one hand and the inner shell on the other hand will the at least one valve means open against this spring force and allow the gas under high pressure to flow from the region between the inner shell and the outer sleeve back into the region of the inner shell. The spring means can thus prevent an opening of the valve means taking place with comparatively balanced pressure situations, so that gas found inside the inner shell could flow out through the valve means into the region between the inner shell and the outer sleeve. This is particularly the case when comparatively high pressures exist that are transferred through an inner shell having a certain elasticity in the manner of a membrane onto the region between the inner shell and the outer sleeve. In case of virtually equal pressures, inadvertent opening could arise in situations in which this is undesirable. Through the spring means in the development according to the invention as described, this is securely and reliably prevented.
According to a further particularly favorable embodiment of the device according to the invention the at least one valve means is designed so that its largest structural expansion is provided inside the inner shell. This structure could be realized, for example, in the form of a spring-loaded flap inside the inner shell that closes openings in the inner shell in the normal state and releases them when the pressure in the region of the inner shell is significantly lower than the pressure in the region between the inner shell and outer sleeve. The structural integration into the inside of the inner shell then allows the conventional structure of the device, for example as a type IV compressed gas storage element, so that the inner shell with incorporated valve means can have fibers soaked with bonding agent wound around it without problems without the valve means or similar projecting over the profile of the inner shell and impairing the structure through a change to the geometric form of the outer sleeve or weakening it in its stability.
According to an alternative or supplementary embodiment of the device according to the invention the at least one valve means is arranged in the region of a connection element for filling/removing the low molecular gas. One or also one of a plurality of valve means can thus be provided in the region of the connection element. In such a connection element there is typically a passage through the different layers of the structure of the device. The connection element is thus connected on its end face both to the inner shell and also the outer sleeve. There is thus also a region of the connection element between these two regions, the region being connected to the region between the inner shell and outer sleeve. A connection to the inside of the inner shell can be produced from this region for example through a borehole or similar. A conventional non-return valve, in particular a spring loaded non-return valve, can then be incorporated into this connection according to the above-described embodiment. At least one of the at least one valve means can be integrated into the structure of the connection elements without notable additional resources.
According to a particularly favorable and advantageous development of the device according to the invention the low molecular gas is hydrogen. In particular when hydrogen is stored, for example under pressures of more than 350 or in particular more than 650 bar, a diffusion through the inner shell of the device inevitably arises so that, particularly with the storage of hydrogen, it is ensured through the material absorbing the hydrogen that unacceptably high hydrogen concentrations in the environment of the device can be prevented during filling. As hydrogen together with oxygen can form an ignitable mixture, a necessary safety-based disconnection can be avoided in case of an unacceptably high hydrogen concentration in the environment of the device.
A particularly preferred use of the device according to the invention in one of the abovementioned embodiments is given when it is a compressed gas storage element that must be comparatively frequently emptied and re-filled. The device can thus be used in particular to store fuel in a vehicle. Such a vehicle, which is operated for example via an internal combustion engine with hydrogen or another low molecular gas, can be equipped particularly efficiently with the device.
Due to the typically relatively high consumption of fuel, lengthy storage times of the fuel in the device are not expected so that the diffusion that is unavoidable at least in case of hydrogen does not constitute a great problem. Since, however, the device for storing the gas must be filled relatively frequently, the solution according to the invention, which prevents an unacceptably high concentration of the gas around the device during filling thereof, constitutes a decisive advantage for this type of application.
A particularly preferred purpose of use thereby lies in the field of fuel cell vehicles, in which the hydrogen can be stored in such devices according to the invention in a particularly user-friendly manner, as these can store an adequate quantity of hydrogen—for example with a pressure level of 700 bar and justifiable volume—in order to achieve a good operating range of the vehicle. Unpleasant emergency disconnections during filling can thereby be avoided reliably and without safety risk.